This invention relates to engineering change (EC) and wire repair of circuit boards, and more particularly to a structure for facilitating ECs and repairs on boards having multichip modules (MCMs) mounted thereon.
In the manufacture of computing hardware with complementary MOS (CMOS) technology, multichip modules (MCMs) are often mounted on circuit boards. A circuit board may be mounted between two aluminum plates for added mechanical strength. These plates, called stiffeners, have windows cut in them to provide space for components. A typical arrangement of an MCM mounted on a board with stiffeners is shown schematically in a cross-sectional view in FIG. 1. In this arrangement, an MCM 1 is mounted on a circuit board 2, with the board placed between a front stiffener 3 and a rear stiffener 4. Windows in the stiffeners provide space for the MCM 1 and other components (for example, power connectors 5).
The level of power dissipation by the components may be high enough to require liquid cooling of the MCMs. This may be accomplished using an arrangement shown in FIG. 2. A cooling liquid circulates through a cooling assembly 20 in contact with a cold plate 21 which in turn is in contact with the MCM 1. The temperature at or near the circuit board 2 in the vicinity of the MCM will thus typically be below the dew point of the ambient air. The cold plate is surrounded by a container of desiccant material 22, to prevent condensation from forming on the MCM or the circuit board. Furthermore, in order to provide a cool air space 23 close to the circuit board and the MCM which is also free of condensation, it is necessary to seal the area of the board surrounding the MCM and to circulate dry air through this space. Since the back side of the circuit board is also expected to be below the dew point of the ambient air, it is necessary to seal the back side of the board as well. This is accomplished by using O-rings 24 between the board and the stiffeners, and a gasket 25 between the cooling assembly 20 and the front stiffener 3. A sealing plate 26 seals the back side of the board, forming an air space 27.
In order to make engineering changes (ECs) and repairs to the modules, it is necessary to run wires between various points on the board surface. However, if the board is cooled and sealed against the stiffeners as described above, the O-ring will prevent external wires from being led from a point inside a module location to a point outside. A conventional method of overcoming this problem is shown schematically in FIG. 3. A typical EC or repair involves making a connection to a pin 31 of the MCM 1, the pin being inserted into a hole 30 in the board 2. Pin 31 is connected to pad 32 on the back side 28 of the board 2. A hole 35 is drilled through the board, inside the O-ring seal; a similar hole 36 is outside the O-ring seal. The inside surfaces of holes 35, 36 are metallized, and a conductor 33, embedded in the board 2, extends between holes 35 and 36. Pads 37 and 38, formed on the board surface, surround holes 35 and 36 respectively and are therefore in electrical contact. The EC/repair may then be made by connecting a wire between pads 32 and 37 inside the sealed air space 27, and connecting an external wire to pad 38. It should be noted that this procedure requires drilling two holes in the board 2 to accommodate each EC or repair. This procedure thus greatly increases the complexity and processing time for each circuit board.
Accordingly, there is a need for an improved circuit board arrangement for MCMs which permits both cooling with O-ring sealing and external wiring for ECs and repairs.
The present invention addresses the above-described need by providing a circuit board structure which facilitates ECs and repairs on the MCM while permitting the MCM to be mounted on the circuit board and hermetically sealed. In accordance with the present invention, this is done by providing a first layer of a conductive material overlying a portion of a surface of the circuit board, and a second layer overlying the first layer and in contact therewith. The second layer has openings therein; bonding pads are formed in the openings of the second layer. The bonding pads are of a conductive material and in contact with the first layer. The multichip module is surrounded by a hermetic seal on the second layer, so that a first bonding pad is inside a hermetically sealed area of the circuit board and a second bonding pad is outside the hermetically sealed area. An engineering change of the multichip module is performed by connecting the multichip module to the first bonding pad by a first wire inside the hermetically sealed area, and by connecting a second wire to the second bonding pad outside the hermetically sealed area. The first layer and the bonding pads are preferably copper. The second layer preferably comprises a solder mask, such as is used in a wave-solder process.